Shroud for a gas turbine engine

ABSTRACT

A gas turbine engine having a rotor blade stage with a plurality of circumferentially spaced rotors, a nozzle stage adjacent the rotor blade stage and including an outer nozzle end, at least one stop, and a shroud. The shroud having a forward end positioned radially outward from the circumferentially spaced rotor blades, and an aft end axially aft of the forward end. The at least one stop confronting at least a portion of the outer nozzle end.

FIELD OF THE INVENTION

The present subject matter relates generally to gas turbine engines andparticularly to features for eliminating leakage between a rotor bladestage and a stator vane stage of a gas turbine engine. Moreparticularly, the present subject matter relates to an elongated shroudfor a gas turbine engine.

BACKGROUND OF THE INVENTION

A gas turbine engine generally includes a fan and a core arranged inflow communication with one another. Additionally, the core of the gasturbine engine generally includes, in serial flow order, a compressorsection, a combustion section, a turbine section, and an exhaustsection. In operation, air is provided from the fan to an inlet of thecompressor section where one or more axial compressors progressivelycompress the air until it reaches the combustion section. Fuel is mixedwith the compressed air and burned within the combustion section toprovide combustion gases. The combustion gases are routed from thecombustion section to the turbine section. The flow of combustion gasesthrough the turbine section drives the turbine section and is thenrouted through the exhaust section, e.g., to atmosphere.

A typical stage of rotor blades, such as a turbine rotor blade stage,includes a shroud positioned radially outward from a platform of eachblade, near the tips of the blades. Similarly, a typical stage of statorvanes or nozzles, such as a turbine nozzle stage, includes an outer bandpositioned radially outward from an inner band, at the radially outerend of each nozzle. Accordingly, at the axial interface between adjacentblade and nozzle stages, a gap exists between the shroud of the bladestage and the outer band of the nozzle stage. As such, fluid flowingwithin or around the stages, such as combustion gases flowing throughthe stages of the turbine section, may leak through the gap between theshroud and the outer band of the nozzle stage, which can impact engineperformance. The axial shroud-outer band interface may pose otherproblems as well.

Therefore, an improved interface between a rotor blade stage and anozzle stage of a gas turbine engine would be desirable. In particular,a shroud that extends over both a rotor blade stage and a nozzle stage,e.g., such that the shroud forms the outer band of the nozzle stage,would be advantageous. More particularly, a shroud over a rotor bladestage that extends axially aft through an adjacent nozzle stage withopenings for nozzles of the nozzle stage would be beneficial. Inaddition, features for sealing the nozzles inserted through the openingsin the shroud to seal the nozzle and the shroud would be desirable.Moreover, a shroud and/or nozzles formed from a ceramic matrix composite(CMC) material would be useful. Further, a method of assembling a gasturbine engine to include a shroud that forms an outer wall of both arotor blade stage and a nozzle stage would be desirable.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one exemplary embodiment of the present disclosure, a shroud segmentfor a gas turbine engine is provided. The gas turbine engine includes arotor blade stage and a nozzle stage. The shroud segment comprises aforward end and an aft end. The forward end defines an outer wall of therotor blade stage, and the aft end defines an outer wall of the nozzlestage. The aft end also defines at least a portion of an openingtherethrough for receipt of a nozzle. The forward end and the aft endform a single, continuous component.

In another exemplary embodiment of the present disclosure, a gas turbineengine is provided. The gas turbine engine comprises a rotor blade stagethat includes a plurality of circumferentially spaced rotor blades, eachrotor blade having a tip. The gas turbine engine also comprises a nozzlestage adjacent the rotor blade stage that includes a plurality ofcircumferentially spaced nozzles, each nozzle extending radially outwardfrom an inner band. Further, the gas turbine engine includes a shroudhaving a forward end positioned radially outward from the tips of therotor blades, and an aft end positioned radially outward from the innerband of the nozzle stage. The shroud extends axially from the forwardend to the aft end. The forward end is positioned near a leading edge ofeach of the plurality of rotor blades and the aft end is positioned neara trailing edge of each nozzle of the plurality of nozzles.

In a further exemplary embodiment of the present disclosure, a method ofassembling a gas turbine engine is provided. The gas turbine engineincludes a rotor blade stage and a nozzle stage. The method comprisespositioning a shroud to form an outer wall of both the rotor blade stageand the nozzle stage.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 provides a schematic cross-sectional view of an exemplary gasturbine engine according to various embodiments of the present subjectmatter.

FIG. 2 provides a close-up, side view of a portion of a turbine sectionof the exemplary gas turbine engine of FIG. 1.

FIG. 3A provides a top view of a shroud segment according to anexemplary embodiment of the present subject matter.

FIG. 3B provides a top view of a shroud segment according to anotherexemplary embodiment of the present subject matter.

FIG. 3C provides a top view of a shroud segment according to anotherexemplary embodiment of the present subject matter.

FIG. 4 provides a top view of a portion of an exemplary shroud formedfrom a plurality of shroud segments as in FIG. 3 and assembled withnozzles of the exemplary gas turbine engine of FIG. 1.

FIG. 5A provides a schematic cross-section view of an outer nozzle endof a nozzle assembled with the exemplary shroud segment of FIG. 3according to an exemplary embodiment of the present subject matter.

FIG. 5B provides a schematic cross-section view of an outer nozzle endof a nozzle assembled with the exemplary shroud segment of FIG. 3according to another exemplary embodiment of the present subject matter.

FIG. 5C provides a schematic cross-section view of an outer nozzle endof a nozzle assembled with the exemplary shroud segment of FIG. 3according to another exemplary embodiment of the present subject matter.

FIG. 6 provides a schematic cross-section view of an inner band segmentof the turbine section of FIG. 2 according to an exemplary embodiment ofthe present subject matter.

FIG. 7 provides a bottom view of an inner nozzle end and seal accordingto an exemplary embodiment of the present subject matter.

FIG. 8 provides a schematic cross-section view of an inner nozzle endassembled with the exemplary inner band segment of FIG. 6 according toan exemplary embodiment of the present subject matter.

FIG. 9 provides a chart illustrating a method for forming a shroudsegment from a ceramic matrix composite (CMC) material according to anexemplary embodiment of the present subject matter.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

As used herein, the terms “first,” “second,” and “third” may be usedinterchangeably to distinguish one component from another and are notintended to signify location or importance of the individual components.The terms “upstream” and “downstream” refer to the relative directionwith respect to fluid flow in a fluid pathway. For example, “upstream”refers to the direction from which the fluid flows, and “downstream”refers to the direction to which the fluid flows. Further, as usedherein, the terms “axial” or “axially” refer to a dimension along alongitudinal axis of an engine. The term “forward” used in conjunctionwith “axial” or “axially” refers to a direction toward the engine inlet,or a component being relatively closer to the engine inlet as comparedto another component. The term “rear” used in conjunction with “axial”or “axially” refers to a direction toward the engine exhaust nozzle, ora component being relatively closer to the engine exhaust nozzle ascompared to another component. The terms “radial” or “radially” refer toa dimension extending between a center longitudinal axis of the engineand an outer engine circumference.

All directional references (e.g., radial, axial, proximal, distal,upper, lower, upward, downward, left, right, lateral, front, back, top,bottom, above, below, vertical, horizontal, clockwise, counterclockwise)are only used for identification purposes to aid the reader'sunderstanding of the present invention, and do not create limitations,particularly as to the position, orientation, or use of the invention.Connection references (e.g., attached, coupled, connected, and joined)are to be construed broadly and may include intermediate members betweena collection of elements and relative movement between elements unlessotherwise indicated. As such, connection references do not necessarilyinfer that two elements are directly connected and in fixed relation toeach other. The exemplary drawings are for purposes of illustration onlyand the dimensions, positions, order and relative sizes reflected in thedrawings attached hereto may vary.

In general, the present subject matter is directed to shroud and nozzleassemblies and features for eliminating an axial interface gap between ashroud and a nozzle. More particularly, in some embodiments, a shroud isformed from a plurality of shroud segments, and each shroud segmentcomprises a forward end and an aft end. The aft end defines at least aportion of an opening therethrough for receipt of a nozzle, and theforward end and the aft end form a single, continuous component. Theforward end is positioned radially outward from tips of a plurality ofrotor blades of a rotor blade stage of a gas turbine engine, and the aftend is positioned radially outward from a plurality of inner bandsegments forming an inner band of a nozzle stage of the gas turbineengine. As such, in an exemplary embodiment, the shroud extends overboth a rotor blade stage and a stator vane stage of a gas turbineengine, which eliminates gaps between a radially outer wall of the rotorblade stage and a radially outer wall of the stator vane stage becausethe radially outer wall of the rotor blade stage and the stator vanestage is a single, unitary component, namely, the shroud.

Referring now to the drawings, wherein identical numerals indicate thesame elements throughout the figures, FIG. 1 is a schematiccross-sectional view of a gas turbine engine in accordance with anexemplary embodiment of the present disclosure. More particularly, forthe embodiment of FIG. 1, the gas turbine engine is a high-bypassturbofan jet engine 10, referred to herein as “turbofan engine 10.” Asshown in FIG. 1, the turbofan engine 10 defines an axial direction A(extending parallel to a longitudinal axis or centerline 12 provided forreference) and a radial direction R. In general, the turbofan 10includes a fan section 14 and a core turbine or gas turbine engine 16disposed downstream from the fan section 14.

The exemplary core turbine engine 16 depicted generally includes asubstantially tubular outer casing 18 that defines an annular inlet 20.The outer casing 18 encases, in serial flow relationship, a compressorsection including a booster or low pressure (LP) compressor 22 and ahigh pressure (HP) compressor 24; a combustion section 26; a turbinesection including a high pressure (HP) turbine 28 and a low pressure(LP) turbine 30; and a jet exhaust nozzle section 32. A high pressure(HP) shaft or spool 34 drivingly connects the HP turbine 28 to the HPcompressor 24. A low pressure (LP) shaft or spool 36 drivingly connectsthe LP turbine 30 to the LP compressor 22.

For the depicted embodiment, fan section 14 includes a fan 38 having aplurality of fan blades 40 coupled to a disk 42 in a spaced apartmanner. As depicted, fan blades 40 extend outward from disk 42 generallyalong the radial direction R. Fan blades 40 and disk 42 are togetherrotatable about the longitudinal axis 12 by LP shaft 36. In someembodiments, the fan blades 40 and disk 42 are rotatable across a powergear box 46 that includes a plurality of gears for stepping down therotational speed of the LP shaft 36 to a more efficient rotational fanspeed.

Referring still to the exemplary embodiment of FIG. 1, disk 42 iscovered by rotatable front nacelle 48 aerodynamically contoured topromote an airflow through the plurality of fan blades 40. Additionally,the exemplary fan section 14 includes an annular fan casing or outernacelle 50 that circumferentially surrounds the fan 38 and/or at least aportion of the core turbine engine 16. It should be appreciated thatnacelle 50 may be configured to be supported relative to the coreturbine engine 16 by a plurality of circumferentially-spaced outletguide vanes 52. Moreover, a downstream section 54 of the nacelle 50 mayextend over an outer portion of the core turbine engine 16 so as todefine a bypass airflow passage 56 therebetween.

During operation of the turbofan engine 10, a volume of air 58 entersturbofan 10 through an associated inlet 60 of the nacelle 50 and/or fansection 14. As the volume of air 58 passes across fan blades 40, a firstportion of the air 58 as indicated by arrows 62 is directed or routedinto the bypass airflow passage 56 and a second portion of the air 58 asindicated by arrows 64 is directed or routed into the LP compressor 22.The ratio between the first portion of air 62 and the second portion ofair 64 is commonly known as a bypass ratio. The pressure of the secondportion of air 64 is then increased as it is routed through the highpressure (HP) compressor 24 and into the combustion section 26, where itis mixed with fuel and burned to provide combustion gases 66.

The combustion gases 66 are routed through the HP turbine 28 where aportion of thermal and/or kinetic energy from the combustion gases 66 isextracted via sequential stages of HP turbine stator vanes 68 that arecoupled to the outer casing 18 and HP turbine rotor blades 70 that arecoupled to the HP shaft or spool 34, thus causing the HP shaft or spool34 to rotate, thereby supporting operation of the HP compressor 24. Thecombustion gases 66 are then routed through the LP turbine 30 where asecond portion of thermal and kinetic energy is extracted from thecombustion gases 66 via sequential stages of LP turbine stator vanes 72that are coupled to the outer casing 18 and LP turbine rotor blades 74that are coupled to the LP shaft or spool 36, thus causing the LP shaftor spool 36 to rotate, thereby supporting operation of the LP compressor22 and/or rotation of the fan 38.

The combustion gases 66 are subsequently routed through the jet exhaustnozzle section 32 of the core turbine engine 16 to provide propulsivethrust. Simultaneously, the pressure of the first portion of air 62 issubstantially increased as the first portion of air 62 is routed throughthe bypass airflow passage 56 before it is exhausted from a fan nozzleexhaust section 76 of the turbofan 10, also providing propulsive thrust.The HP turbine 28, the LP turbine 30, and the jet exhaust nozzle section32 at least partially define a hot gas path 78 for routing thecombustion gases 66 through the core turbine engine 16.

It will be appreciated that, although described with respect to turbofan10 having core turbine engine 16, the present subject matter may beapplicable to other types of turbomachinery. For example, the presentsubject matter may be suitable for use with or in turboprops,turboshafts, turbojets, industrial and marine gas turbine engines,and/or auxiliary power units.

Referring now to FIG. 2, a schematic view is provided of the HP turbine28 of the turbine section of core turbine engine 16, which is locateddownstream from combustion section 26. The combustion section 26generally includes a combustor defining a combustion chamber 80; amixture of fuel and air is combusted within the combustion chamber togenerate a flow of combustion gases therethrough. Downstream of thecombustion section 26, the HP turbine 28 includes a plurality of turbinecomponent stages. Each turbine component stage comprises a plurality ofturbine components that define and/or are positioned within the hot gaspath 78 through which the combustion gases flow.

More particularly, for the embodiment depicted in FIG. 2, HP turbine 28includes a plurality of turbine nozzle stages, as well as one or morestages of turbine rotor blades. Specifically, the HP turbine 28 includesa first turbine nozzle stage 100 and a second turbine nozzle stage 102,each configured to direct a flow of combustion gasses therethrough.Notably, the first turbine nozzle stage 100 is located immediatelydownstream from the combustion section 26, and thus may also be referredto as a combustor discharge nozzle stage having a plurality ofcombustion discharge nozzle sections.

The first turbine nozzle stage 100 includes a plurality of turbinenozzle sections 104 spaced along the circumferential direction M (FIG.3). Each first turbine nozzle section 104 forming the first turbinenozzle stage 100 includes a first stage turbine nozzle 106 positionedwithin the hot gas path 78. Further, each nozzle section 104 includes aninner band segment 110 defining an inner wall of the nozzle section 104and an outer band segment 112 defining an outer wall of the nozzlesection 104, with nozzle 106 extending generally along the radialdirection R from inner band segment 110 to outer band segment 112. Innerband segment 110 of the first nozzle section 104 defines a cold side 114and an opposite hot side 116; hot side 116 is exposed to and at leastpartially defining the hot gas path 78. Similarly, outer band segment112 of the first nozzle section 104 defines a cold side 118 and anopposite hot side 120; hot side 120 is exposed to and at least partiallydefining the hot gas path 78. Together, the plurality of first turbinenozzle sections 104 define the first turbine nozzle stage 100, with aninner band defined by the plurality of inner band segments 110 of nozzlesections 104, an outer band defined by the plurality of outer bandsegments 112 of nozzle sections 104, and a plurality of nozzles 106extending from the inner band to the outer band.

Located immediately downstream of the first turbine nozzle stage 100 andimmediately upstream of the second turbine nozzle stage 102, the HPturbine 28 includes a first stage 122 of turbine rotor blades 124. Firststage 122 of turbine rotor blades 124 includes a plurality of turbinerotor blades 124 spaced along the circumferential direction M (FIG. 3)and a first stage rotor 126. Each turbine rotor blade 124 has an airfoil123 extending axially between a leading edge 125 and a trailing edge127. Further, each rotor blade 124 is attached to the first stage rotor126. Although not depicted, the first stage turbine rotor 126 is, inturn, connected to the HP shaft 34 (FIG. 1). In such manner, the turbinerotor blades 124 may extract kinetic energy from the flow of combustiongases through the hot gas path 78 defined by the HP turbine 28 asrotational energy applied to the HP shaft 34. Core gas turbine engine 16additionally includes a shroud 128 exposed to and at least partiallydefining hot gas path 78. Shroud 128 is described in greater detailbelow.

Similar to the plurality of nozzle sections 104 forming the firstturbine nozzle stages 100, a radially inner portion of each turbinerotor blade 124 includes a wall or platform 130. The platform 130 ofeach turbine rotor blade 124 defines a cold side 132, as well as anopposite hot side 134 exposed to and at least in part defining the hotgas path 78. Additionally, each turbine rotor blade 124 includes a tip136 at a radially outer portion of the blade. Shroud 128 may radially bespaced apart from the blade tips 136 such that a radial or clearance gapCL is defined between tips 136 and shroud 128. That is, shroud 128 ispositioned radially adjacent blades 124 to define an outer wall of therotor blade stage 122. Shroud 128 includes a radially inner surface 138opposite a radially outer surface 140. Outer surface 140 is a cold sidesurface of shroud 128, and inner surface 138, which is exposed to and atleast in part defines the hot gas path 78, is a hot side surface ofshroud 128. Accordingly, the clearance gap CL is defined between bladetips 136 and the hot side surface 138 of shroud 128. As shown in FIG. 2,shroud 128 is tightly configured relative to the blades 124 so that theshroud 128 defines an outer radial flow path boundary for the hotcombustion gas flowing through the turbine 16. It is generally desirableto minimize the clearance gap CL between the blade tips 136 and theturbine shroud 128, particularly during cruise operation of the turbofan10, to reduce leakage from the hot gas path 78 over the blade tips 136and through the clearance gap CL.

As depicted in FIG. 2, shroud 128 extends axially aft through secondturbine nozzle stage 102. As such, shroud 128 extends within the firstrotor blade stage 122 and the second turbine nozzle stage 102. Similarto the first turbine nozzle stage 100, the second turbine nozzle stage102 includes a plurality of second stage turbine nozzles 108 positionedwithin the hot gas path 78. More particularly, second turbine nozzlestage 102 includes a plurality of inner band segments 142 forming aninner wall of second turbine nozzle stage 102, and shroud 128 ispositioned to form an outer wall of second turbine nozzle stage 102.Each second stage turbine nozzle 108 extends generally along the radialdirection R from an inner band segment 142 through an opening 144 (FIG.3) in the shroud 128. Each inner band segment 142 of the second turbinenozzle stage 102 defines a cold side 146 and an opposite hot side 148,which is exposed to and at least partially defines the hot gas path 78.Moreover, the plurality of inner band segments 142 together defines aninner band of second turbine nozzle stage 102. Further, radially outwardfrom inner band segment 142, inner surface 138 of shroud 128 defines ahot side of second turbine nozzle stage 102 and outer surface 140 ofshroud 128 defines a cold side. As discussed above, the inner surface138 of shroud 128 is exposed to and at least partially defines the hotgas path 78. Accordingly, inner surface 138 of shroud 128 at leastpartially defines the hot gas path 78 through first turbine blade stage122 and second turbine nozzle stage 102.

Each nozzle 108 includes an airfoil 160 having a concave pressure side162 opposite a convex suction side 164 (FIG. 7). As such, pressure andsuction sides 162, 164 of each airfoil 160 generally define an airfoilshape. Opposite pressure and suction sides 162, 164 of each airfoil 160extend radially along a span from a radially inner nozzle end 166 to aradially outer nozzle end 168. Moreover, pressure and suction sides 162,164 of airfoil 160 extend axially between a leading edge 170 and anopposite trailing edge 172, and pressure and suction sides 162, 164define an outer surface 174 of airfoil 160.

Shroud 128 generally forms a ring or shroud around a stage of turbinerotor blades and a stage of turbine nozzles adjacent the stage ofturbine rotor blades, i.e., each shroud 128 extends circumferentiallyabout the longitudinal engine axis 12 proximate a turbine rotor bladestage and a turbine nozzle stage. In the depicted exemplary embodiment,shroud 128 is an annular shroud that extends circumferentially aroundfirst stage 122 of turbine rotor blades 124 and second turbine nozzlestage 102; that is, shroud 128 extends about first turbine rotor bladestage 122 and second turbine nozzle stage 102, which is immediatelydownstream, i.e., axially aft, of first stage 122 of turbine rotorblades 124.

In some embodiments, shroud 128 may be formed as a continuous, unitary,or seamless ring. However, as shown in FIGS. 3 and 4, in otherembodiments shroud 128 may be formed from a plurality of shroud segments150. Each shroud segment 150 includes a forward end 152 and an aft end154. Further, each shroud segment 150 is a single, continuous componentthat extends axially from its forward end 152 to its aft end 154. Thus,in exemplary embodiments such as the embodiment shown in FIG. 2, shroud128 may include a plurality of shroud segments 150 positioned next toone another along the circumferential direction to form generallyannular shroud 128 around first turbine rotor blade stage 122 and secondturbine nozzle stage 102.

Further, although described herein with respect to HP turbine 28, itshould be noted that shroud 128 may additionally be utilized in asimilar manner in the low pressure compressor 22, high pressurecompressor 24, and/or LP turbine 30. Accordingly, shrouds as disclosedherein are not limited to use in HP turbines but rather may be utilizedin any suitable section of a turbofan 10, e.g., in any suitable sectionof core turbine engine 16 of turbofan 10.

Referring to FIG. 2, the forward end 152 of each shroud segment 150 ispositioned radially outward from the tips 136 of rotor blades 124, aswell as the platforms 130 of blades 124. Moreover, the forward end 152of each shroud segment 150 is positioned axially near the leading edges125 of rotor blades 124. As such, the forward end 152 of each shroudsegment 150 is positioned to define an outer wall segment of rotor bladestage 122, and the clearance gap CL is defined between each of theplurality of blades 124 and inner surface 138 of shroud segment 150. Theaft end 154 is positioned radially outward from the inner band segments142 of second turbine nozzle stage 102 and axially near the trailingedges 172 of nozzles 108. Accordingly, the aft end 154 of each shroudsegment 150 is positioned to define an outer wall segment of nozzlestage 102. It will be understood that each shroud segment 150 includes aradially inner surface 138 and a radially outer surface 140 as describedabove with respect to shroud 128.

Moreover, as shown in FIG. 3A, each shroud segment 150 defines a firstside 156 and a second side 158. Each of the first side 156 and secondside 158 extend axially from the forward end 152 to the aft end 154.Additionally, in the illustrated embodiment, each of the first side 156and the second side 158 extend at a non-orthogonal angle with respect tothe forward end 152 and the aft end 154 of the respective shroud segment150. That is, first and second sides 156, 158 are angled with respect toforward and aft ends 152, 154 of shroud segment 150. More particularly,the first side 156 extends at a first non-orthogonal angle α withrespect to forward end 152 or aft end 154, and the second side 158extends at a second non-orthogonal angle β with respect to forward end152 or aft end 154. As illustrated in FIG. 3, first side 156 extends atfirst non-orthogonal angle α with respect to forward end 152, and thesecond side 158 extends at second non-orthogonal angle β with respect toforward end 152. Angles α and β generally may be complementary to oneanother such that the first side 156 of one shroud segment 150 may bepositioned adjacent the second side 158 of another shroud segment toform shroud 128, as shown in FIG. 4. Of course, in other embodiments,first and second sides 156, 158 may not be angled with respect toforward and aft ends 152, 154.

A turbine stator, such as second turbine nozzle stage 102, may be formedby a plurality of segments that are abutted at circumferential sides toform a complete ring about engine centerline 12. In the depictedembodiment of FIG. 2, the turbine stator 102 is formed from a pluralityof shroud segments 150 abutted at their circumferential first and secondsides 156, 158 and a plurality of inner band segments 142 abutted attheir circumferential sides, with second stage nozzles 108 extendingfrom the inner band segments 142 through openings 144 in shroud segments150. As such, inner band segments 142 form an inner band of the secondturbine nozzle stage 102 and the shroud segments 150 form an outer bandof the second turbine nozzle stage 102. As described above, the innerband defines an inner wall of the nozzle stage 102 and the outer banddefines an outer wall of the nozzle stage 102. It will be understoodthat, in this exemplary embodiment, because shroud 128 (formed by shroudsegments 150) extends axially from the first turbine rotor stage 122through the second turbine nozzle stage 102, and more particularly, froma forward end generally near the leading edges of the blades of theturbine rotor stage to an aft end generally near the trailing edges ofthe nozzles of the turbine nozzle stage, the outer band of secondturbine nozzle stage 102 is integral with the shroud of the firstturbine rotor stage 122.

As previously stated, each nozzle 108 of second turbine nozzle stage 102extends from an inner band segment 142 and through a correspondingopening 144 in shroud 128. More particularly, as shown in FIGS. 3Athrough 3C, the aft end 154 of each shroud segment 150 defines at leasta portion of an opening 144 therethrough for receipt of a second stagenozzle 108. As such, each opening 144 generally may be airfoil-shaped,i.e., each opening 144 generally may correspond to the shape of theairfoil portion 160 of second stage turbine nozzle 108 such that eachopening 144 has a shape complementary to the airfoil shape. In someembodiments, such as the embodiment of FIG. 3A, each shroud segment 150may define one opening 144, but in other embodiments, shroud segments150 may define more than one opening 144, such as two openings 144 asshown in FIG. 3B, or three or more openings 144. In still otherembodiments, such as the embodiment depicted in FIG. 3C, a first shroudsegment 150 may define a portion of an opening 144, and a second,adjacent shroud segment 150 may define the remainder of the opening 144such that when the first and second shroud segments are abutted againstone another, the first and second shroud segments together define theopening 144. Of course, shroud 128 may comprise any combination ofshroud segment 150 configurations, i.e., not all shroud segments 150need define the same number of openings 144.

Further, as depicted in FIGS. 2 and 4, when a nozzle 108 is receivedwithin an opening 144, the outer nozzle end 168 of the nozzle 108extends radially outward beyond the outer surface 140 of the shroudsegment 150. Moreover, the radially inner nozzle end 166 of a nozzle 108is received within a depression or cavity 176 (FIG. 6) in an inner bandsegment 142. Thus, nozzles 108 of second turbine nozzle stage 102 extendradially from inner band segments 142 through the openings 144 definedin the aft ends 154 of the plurality of shroud segments 150.

As illustrated in FIGS. 2 through 4, each shroud segment 150 may includea plurality of features for mounting the shroud 128 within HP turbinesection 28 of core turbine engine 16, as well as a plurality of featuresfor retaining nozzle 108 within opening 144 in the shroud segment 150.Additionally or alternatively, features also may be provided formaintaining the relative positions of nozzle 108 and shroud segment 150within the turbine assembly. For example, shroud segment 150 includesrails 178, which also may be referred to as hanger pin mounts or simplyhangers, for coupling the shroud segment 150 to, e.g., outer casing 18of core turbine engine 16 as shown in FIG. 2 (any reciprocal mountinghardware of casing 18 is omitted for clarity). In the embodiment shownin FIGS. 2 through 4, each rail 178 extends outward along the radialdirection R from the radially outer surface 140 of shroud segment 150and also extends circumferentially along the circumferential directionM. Each rail 178 defines one or more apertures 180, each aperture 180configured for receipt of a mounting pin 182 to couple the shroudsegment 150 to a corresponding mount or other appropriate portion of,e.g., casing 18. Of course, other types or forms of fasteners may beused to attach shroud segments 150 to an appropriate component of engine16.

Rails 178 extend from the forward end 152 of shroud segment 150. Theexemplary shroud segment 150 also includes a pin mount 184 extendingalong the radial direction R from the outer surface 140 of shroudsegment 150 at the aft end 154 of the shroud segment 150. Similar torails 178, pin mount 184 also may extend along the circumferentialdirection M and may define one or more apertures 180, each aperture 180configured for receipt of a mounting pin 182 to couple the shroudsegment 150 to a corresponding mount or other appropriate portion of,e.g., casing 18. As previously discussed, a fastener other than mountingpins 182 also may be used to mount shroud segments 150 within engine 16.Further, as shown in FIGS. 2, 3, and 4, each shroud segment 150 includesa pair of rails 178, spaced apart from one another along the axialdirection A, and a pin mount 184 for attaching shroud segments 150, andthereby shroud 128, to the casing 18 or other component of the turbofan10. Nevertheless, shroud segments 150 may include any suitableprojection(s) or other feature(s) for mounting shroud 128 within theassembly forming HP turbine section 28.

As further illustrated in FIG. 4, shroud segments 150 optionally mayinclude a radial retainer 186, which, e.g., may help engage or seat aseal 188 (FIGS. 5A-5C) between nozzle 108 and shroud segment 150. Theseal 188, which may be referred to as nozzle-shroud seal 188, isdescribed in greater detail below. Retainer 186 extends generallycircumferentially along the circumferential direction M between a firstretainer mount 190 a adjacent first side 156 of shroud segment 150 and asecond retainer mount 190 b defined adjacent second side 158 of shroudsegment 150. Similar to rails 178 and pin mount 184, first and secondretainer mounts 190 a, 190 b extend along the radial direction R fromthe outer surface 140 of shroud segment 150. Moreover, each retainermount 190 a, 190 b define an aperture 192 for receipt of an end 194 ofretainer 186. More particularly, first retainer mount 190 a defines anaperture 192 for receipt of a first end 194 a of retainer 186, andsecond retainer mount 190 b defines an aperture 192 for receipt of asecond end 194 b of retainer 186. The radial retainer 186, e.g., mayassist in holding nozzle 108 in place during assembly of the HP turbinesection 28 or, as mentioned above, may help engage the seal 188 betweennozzle 108 and shroud segment 150.

Turning to FIGS. 5A through 5C, various configurations of nozzle-shroudseals 188 are illustrated. Seals 188 help reduce leakage of combustiongases 66 from the hot gas path 78 between nozzles 108 and shroud 128(formed by shroud segments 150). In exemplary embodiments, turbine 16includes a plurality of nozzle-shroud seals 188 such that a seal 188extends along an entire interface 196 between a nozzle 108 and a shroudsegment 150, i.e., a seal 188 is provided for each nozzle-shroudinterface 196. More specifically, each opening 144 defines a perimeter,which may be generally airfoil-shaped as described above, and nozzle 108interfaces with a shroud segment 150, or adjacent first and secondshroud segments 150 when opening 144 is partially defined by each of thefirst and second shroud segments 150, along the perimeter. Thus,nozzle-shroud seal 188 is positioned to provide a seal along theinterface 196 between nozzle 108 and shroud segment(s) 150. For example,outer nozzle end 168 may define a flange 198 extending about outernozzle end 168 (FIGS. 2 and 4), and flange 198 may have a contactsurface 200 extending adjacent outer surface 140 of shroud segment 150.In one embodiment, as shown in FIG. 5A, nozzle-shroud seal 188 ispositioned between outer surface 140 of shroud segment 150 and contactsurface 200 of flange 198 of nozzle 108. In another embodiment,illustrated in FIG. 5B, outer nozzle end 168 may define a pocket 202adjacent contact surface 200, and nozzle-shroud seal 188 may bepositioned within pocket 202 to provide a seal between flange 198 ofnozzle 108 and outer surface 140 of shroud segment 150. In yet anotherembodiment, depicted in FIG. 5C, shroud segment 150 may define a pocket204 adjacent opening 144, and nozzle-shroud seal 188 may be positionedwithin pocket 204 to provide a seal between airfoil portion 160 ofnozzle 108 and shroud segment 150. Of course, nozzle-shroud seal 188 maybe positioned in other locations as well to help reduce the amount ofcombustion gases 66 that escape from hot gas path 78 through theinterface 196 between nozzle 108 and shroud segment 150.

As further shown in FIGS. 5A through 5C, nozzle-shroud seal 188 may haveany appropriate cross-section and size and may be any appropriate typeof seal for reducing the loss of combustion gases 66 through thenozzle-shroud interface 196. For example, seal 188 may have a generallyrectangular cross-section as shown in FIG. 5A or a generally circularcross-section as shown in FIGS. 5B and 5C. Further, in variousembodiments, each nozzle-shroud seal 188 may be a ring seal, wire seal,C-seal, rope seal, piston seal, or any other suitable type of seal.Additionally, any combination of suitable seals may be used for theplurality of seals 188, i.e., each seal 188 need not be the same type orsize of seal or have the same cross-sectional shape.

Turning now to FIGS. 6-8, a seal 206 also may be included between nozzle108 and inner band segment 142; as such, seal 206 may be referred to asnozzle-band seal 206. Seal 206 helps reduce leakage between nozzles 108and the inner band of second turbine nozzle stage 102, which is formedby inner band segments 142. In exemplary embodiments, turbine 16includes a plurality of nozzle-band seals 206 such that a seal 206extends along an entire interface 208 between a nozzle 108 and an innerband segment 142, i.e., a seal 206 is provided for each nozzle-bandinterface 208. More specifically, depression 176 defined in each innerband segment 142 generally defines an airfoil shape that correspondswith the shape of inner nozzle end 166, and the inner end 166 of anozzle 108 interfaces with a respective inner band segment 142 when thenozzle 108 is received within the depression. Thus, nozzle-band seal 206is positioned to provide a seal along the interface 208 between nozzle108 and inner band segment 142. For example, as illustrated in FIG. 8,nozzle-band seal 206 may be positioned between the radially innermostsurface 210 of inner nozzle end 166 and a radially innermost surface 212of inner band segment 142 defining depression 176. In other embodiments,nozzle-band seal 206 may be positioned between pressure and suctionsides 162, 164 of airfoil 160 at inner nozzle end 166 and a side surface214 of inner band segment defining depression 176. Alternatively,nozzle-band seal 206 may be positioned in any appropriate location, andany suitable size, shape, type, and/or number of nozzle-band seal 206may be used.

In some embodiments, nozzle-shroud seals 188 and/or nozzle-band seals206 may be constructed from a suitable metallic material, such as anickel or cobalt alloy. Suitable nickel and cobalt alloys include RENE41® Alloy produced by General Electric Co. of Schenectady, N.Y., USA;HAYNES® alloy 188 produced by Haynes International of Kokomo, Ind., USA;and UDIMET® alloy L-605 produced by Special Metal Corporation of NewHartford, N.Y., USA. However, any suitable metallic material may be usedfor seals 188 and/or seals 206.

Referring back to FIGS. 2 and 4, one or more radial, axial, and/orcircumferential stops also may be used with shroud segments 150 andnozzles 108. For example, an axial stop 216 a may be positioned againstouter nozzle end 168 toward the trailing edge 172 of nozzle 108, and acircumferential stop 216 c may be positioned against outer nozzle end168 toward suction side 164 of nozzle 108. Stops 216 a, 216 c may, e.g.,help load nozzle 108 into case 18. Stops 216 a, 216 c also may helpmaintain the relative axial positions of nozzle 108 and shroud segment150 and, in at least some embodiments, may provide some sealing againstthe loss of combustion gases 66 from hot gas path 78.

Although the shroud 128 is described above as forming an outer wall ofboth the first turbine rotor blade stage 122 and the second turbinenozzle stage 102, in other embodiments, the shroud 128 may be used toform an outer wall of other stages of the turbine section or may be usedin the compressor section of turbofan engine 10. For example, the shroud128 may be configured to form an outer wall of the first turbine nozzlestage 100 and first turbine rotor blade stage 122. In such embodiments,the shroud may extend axially from a forward end positioned generallynear leading edges of nozzles 106 of the first turbine nozzle stage 100to an aft end positioned generally near trailing edges 127 of blades 124of the first turbine rotor blade stage 122. As another example, theshroud 128 may be configured to form an outer wall of the second turbinenozzle stage 102 and a second turbine rotor blade stage located axiallyaft of the second turbine nozzle stage 102. In such embodiments, theshroud may extend axially from a forward end positioned generally nearleading edges 170 of nozzles 108 of the second turbine nozzle stage 102to an aft end positioned generally near trailing edges of blades of thesecond turbine rotor blade stage. Of course, in appropriate embodiments,the shroud 128 may be elongated to form an outer wall of more than twostages of the turbine or compressor sections. Further, it will beunderstood that in embodiments in which shroud 128 is configured toextend through stages other than the first turbine rotor blade stage 122and second turbine nozzle stage 102 as described above, the shroud 128may be otherwise configured as described above. That is, the shroud 128may define openings 144, rails 178, pin mounts 184, and the like,although the relative positions of such features may be different basedon which stages to which the shroud is adjacent. As one example,openings 144 would be defined in the forward end of shroud 128, ratherthan the aft end, in embodiments in which shroud 128 extends from anozzle stage through a blade stage.

Accordingly, a method of assembling turbofan engine 10 may includepositioning one of a rotor blade stage and a nozzle stage immediatelyupstream of the other of the rotor blade stage or nozzle stage such thatone stage is an upstream stage and the other stage is a downstreamstage. The method also may include positioning a shroud 128 to form anouter wall of both the rotor blade stage and the nozzle stage of theengine. When positioned to form the outer wall of both stages, theshroud extends axially from near the leading edges of the airfoils ofthe upstream stage to near the trailing edges of the airfoils of thedownstream stage. Further, the method may include attaching the shroud128 to a casing of the engine, such as casing 18 of turbofan engine 10;providing seals between a plurality of nozzles and the shroud 128 and/orbetween a plurality of nozzles and an inner band of the nozzle stage;and/or providing one or more stops as previously described. As anexample of such a method of assembly, a shroud 128 may be positioned toform an outer wall of both first turbine rotor blade stage 122 andsecond turbine nozzle stage 102 as described above, where first turbinerotor blade stage 122 is positioned immediately upstream of secondturbine nozzle stage 102 such that blade stage 122 is an upstream stageand nozzle stage 102 is a downstream stage. In such embodiments, theshroud 128 extends axially from near the leading edges 125 of airfoils123 of the upstream rotor blade stage 122 to near the trailing edges 172of airfoils 160 of the downstream nozzle stage 102. The shroud 128 maybe attached to casing 18 using rails 178, pin mounts 184, etc., andnozzle-shroud seals 188, nozzle-band seals 206, and/or stops 216 a, 216c may be provided, e.g., to help stop leakage of combustion gases 66from hot gas path 78, to help load nozzles 108 into casing 18, and/or tohelp maintain the relative positions of nozzles 108 and shroud 128. Ofcourse, as described, shroud 128 may be formed from a plurality ofshroud segments 150, such that the method comprises positioning theplurality of shroud segments 150 to form an outer wall of both a rotorblade stage and a nozzle stage.

In some embodiments, components of turbofan engine 10, particularlycomponents within hot gas path 78, may comprise a ceramic matrixcomposite (CMC) material, which is a non-metallic material having hightemperature capability. Exemplary CMC materials utilized for suchcomponents may include silicon carbide, silicon, silica, or aluminamatrix materials and combinations thereof. Ceramic fibers may beembedded within the matrix, such as oxidation stable reinforcing fibersincluding monofilaments like sapphire and silicon carbide (e.g.,Textron's SCS-6), as well as rovings and yarn including silicon carbide(e.g., Nippon Carbon's NICALON®, Ube Industries' TYRANNO®, and DowCorning's SYLRAMIC®), alumina silicates (e.g., Nextel's 440 and 480),and chopped whiskers and fibers (e.g., Nextel's 440 and SAFFIL®), andoptionally ceramic particles (e.g., oxides of Si, Al, Zr, Y, andcombinations thereof) and inorganic fillers (e.g., pyrophyllite,wollastonite, mica, talc, kyanite, and montmorillonite). As furtherexamples, the CMC materials may also include silicon carbide (SiC) orcarbon fiber cloth.

CMC materials may be used for various components of the engine, forexample, turbine nozzles and/or airfoils in the compressor, and/or fanregions. With respect to the present subject matter, shroud segments 150or nozzles 108, or both shroud segments 150 and nozzles 108, may be madefrom CMC materials. That is, in one embodiment, shroud segments 150 maybe made from a CMC material, and nozzles 108 may be made from a metallicmaterial. In another embodiment, shroud segments 150 and nozzles 108 maybe made from a CMC material, and in some embodiments, shroud segments150 and nozzles 108 may be integrally formed from a CMC material suchthat a shroud segment 150 and one or more nozzles 108 are a single,continuous CMC component. In still another embodiment, shroud segments150 may be made from a metallic material, and nozzles 108 may be madefrom a CMC material. In yet another embodiment, shroud segments 150 andnozzles 108 may be made from a metallic material, and in appropriateembodiments, shroud segments 150 and nozzles 108 may be integrallyformed from a metallic material. Other materials may be used as well.Further, it will be appreciated that, as previously stated, shroud 128may be formed as a single piece component rather than from a pluralityof shroud segments 150, and in such embodiments, shroud 128 may beformed from a metallic, CMC, or other appropriate material.

Of course, other components of turbofan 10, including other componentsof HP turbine section 28, may be made from CMC materials such thatturbofan 10 may comprise components formed from any combination ofmaterials. As a particular example, inner band segments 142 may be madefrom a CMC material, and as a further example, inner band segments 142and nozzles 108 may be integrally formed from a CMC material such thatan inner band segment 142 and one or more nozzles 108 are a single,continuous CMC component. However, inner band segments 142, or the innerband of second nozzle stage 102 if the inner band is formed as a singlepiece component rather than from a plurality of segments 142, also maybe formed from a metallic or other material, and in appropriateembodiments, may be integrally formed with nozzles 108 from a metallicor other suitable material.

FIG. 9 provides a chart illustrating a method 900 for forming a shroudsegment 150 from a CMC material to form a CMC shroud segment 150,according to an exemplary embodiment of the present subject matter. Asshown at 902 in FIG. 9, a plurality of plies of a CMC material forforming the CMC shroud 150 may be laid up to define a desired shape.Preferably, the plies forming shroud segment 150 contain continuous CMCfibers along their lengths. Continuous fiber CMC plies can help avoidrelying on the interlaminar capability of the material to resiststresses on the component.

During the layup, a desired component shape may be generally defined;the component shape may be finally defined after the plies are processedand machined as needed. The CMC plies may be laid up on a layup tool,mandrel, mold, or other appropriate device for supporting the pliesand/or for defining the desired shape. In one embodiment, laying up theCMC plies may comprise layering a plurality of CMC plies defining theforward end 152 of shroud segment 150 with a plurality of CMC pliesdefining the aft end 154 of shroud segment 150. The plies may be layeredby alternating plies such that the forward end plies are interspersedwith the aft end plies. That is, laying up the plurality of CMC plies toform shroud segment 150 may include interspersing forward and aft endCMC plies. Interspersing the plies for forming various portions ofshroud segment 150 integrates the various portions such that theresultant CMC component is an integral component.

In some embodiments, multiple layups or preforms may be laid up togetherto form a preform assembly. More particularly, the layup portion ofmethod 900 depicted at 902 in FIG. 9 may include laying up multiplepreforms, layups, and/or plies to form a shroud segment preform. Forexample, the layup preforming step may comprise layering multiple pliesor structures, such as plies pre-impregnated with matrix material(prepreg plies), prepreg tapes, or the like, to form a desired shape ofthe resultant CMC component. The layers are stacked to form a layup orpreform, which is a precursor to the CMC component, e.g., shroud segment150.

After the CMC plies and/or other CMC structures are laid up to form thelayup, the layup may be processed, e.g., compacted and cured in anautoclave, as shown at 904 in FIG. 9. After processing, the layup is agreen state component, i.e., a green state shroud segment 150. The greenstate component is a single piece component, i.e., curing the plies 124produces a unitary component formed from a continuous piece of CMCmaterial.

Various methods, techniques, and/or processes may be used to form anopening 144, or at least a portion of an opening 144, in a shroudsegment 150. For example, in some embodiments, at least a portion of anopening 144 may be defined by cutting each individual ply forming theaft end 154 of shroud segment 150 before the plies are laid up to formshroud segment 150. The plies may be cut, e.g., using a precision Gerbercutter by Gerber Technology of Tolland, Connecticut. In otherembodiments, another type of cutter or other means may be used to formcut-outs in the plies to define the opening 144. Alternatively oradditionally, the shroud segment may be processed in an autoclave, e.g.,at a reduced temperature relative to a standard autoclave cycle suchthat the green state component retains some flexibility and malleabilityafter autoclaving, which can assist in further manipulation of thecomponent, such as defining opening 144. For example, at least a portionof an opening 144 may be machined in the green state shroud segment 150,and the malleability of the green state component may help in machiningthe opening. Opening 144, or at least a portion thereof, may be formedin the green state shroud segment 150 using one or more of laserdrilling, electric discharge machining (EDM), precision machining,cutting, or other machining methods. In other embodiments, the CMC pliesmay be processed using a standard autoclave cycle and then the greenstate component may be machined as described to define at least aportion of opening 144. Of course, other apertures or openings in shroudsegment 150, e.g., cooling passages or the like, may be defined bycutting the CMC plies that are laid up to define the component or bymachining the green state component as described, or using any othersuitable technique.

Next, the green state component may undergo firing (or burn-out) anddensification, illustrated at 906 and 908 in FIG. 9, to produce a finalunitary CMC shroud segment 150. In an exemplary embodiment of method900, the green state component is placed in a furnace with silicon toburn off any mandrel-forming materials and/or solvents used in formingthe CMC plies, to decompose binders in the solvents, and to convert aceramic matrix precursor of the plies into the ceramic material of thematrix of the CMC shroud segment 150. The silicon melts and infiltratesany porosity created with the matrix as a result of the decomposition ofthe binder during burn-off/firing. However, densification may beperformed using any known densification technique including, but notlimited to, Silcomp, melt-infiltration (MI), chemical vapor infiltration(CVI), polymer infiltration and pyrolysis (PIP), and oxide/oxideprocesses. In one embodiment, densification and firing may be conductedin a vacuum furnace or an inert atmosphere having an establishedatmosphere at temperatures above 1200° C. to allow silicon or otherappropriate material or materials to melt-infiltrate into the CMCcomponent.

After firing and densification, as shown at 910 in FIG. 4, the CMCshroud segment 150 may be finished. For example, the component may befinish machined, if and as needed, to finish the component. Additionallyor alternatively, an environmental barrier coating (EBC) may be appliedto CMC shroud segment 150 to finish the component. It will beappreciated that the CMC shroud segment 150 produced using the foregoingmethod 900 comprises the features of shroud segment 150 previouslydescribed, e.g., forward and aft ends 152, 154, with at least a portionof an opening 144 defined in the aft end 154.

Method 900 is provided by way of example only. For example, otherprocessing cycles, e.g., utilizing other known methods or techniques forcompacting and/or curing CMC plies, may be used. Further, CMC shroudsegment 150 may be post-processed or densified using a melt-infiltrationprocess or a chemical vapor infiltration process, or CMC shroud segment150 may be a matrix of pre-ceramic polymer fired to obtain a ceramicmatrix. Alternatively, any combinations of these or other knownprocesses may be used.

Of course, as described above, other components of turbofan 10, such assecond stage turbine nozzles 108, may be fabricated using CMC materials.In an exemplary embodiment, nozzles 108 may be CMC nozzles 108 formedusing a method similar to method 900. For example, a plurality of CMCplies may be laid up, with the layup of plies defining a desired shapeof nozzle 108. Then, the nozzle 108 may be processed, e.g., compactedand cured in an autoclave, to form a green state CMC nozzle 108. In someembodiments, the green state nozzle 108 may be machined, e.g., using alaser, precision, EDM, or other suitable machining process, to definevarious features of nozzle 108. As an example, for a nozzle 108incorporating a pocket 202 defined in the flange portion 198 of outernozzle end 168, the pocket 202 may be machined in the green state nozzle108. After processing and any green state machining, the nozzle 108 maybe fired and densified to form a final, unitary CMC nozzle 108. Forexample, the CMC nozzle 108 may be fired in a furnace and densifiedusing a melt-infiltration process whereby the fired CMC nozzle 108 ismelt-infiltrated with, e.g., silicon. After firing and densification,CMC nozzle 108 also may be finish machined and/or coated with, e.g., anEBC, as desired. Other processes, e.g., a CVI or PIP densificationrather than MI densification, may be used as well to form a CMC nozzle108.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

1-20. (canceled)
 21. A gas turbine engine defining a longitudinal axis,the gas turbine engine comprising: a rotor blade stage, the rotor bladestage including a plurality of circumferentially spaced rotor bladeswith respect to the longitudinal axis, each rotor blade having a tip; anozzle stage adjacent the rotor blade stage, and including an outernozzle end; at least one stop confronting at least a portion of theouter nozzle end and configured to axially or circumferentially positionthe nozzle stage; a shroud having: a forward end positioned radiallyoutward from each tip of the circumferentially spaced rotor blades; andan aft end positioned axially aft of the forward end with respect to thelongitudinal axis, wherein the outer nozzle end extends radially outwardbeyond an outer surface of the shroud.
 22. The gas turbine engine ofclaim 21, wherein the outer nozzle end includes a flange extendingacross at least a portion of the outer nozzle end.
 23. The gas turbineengine of claim 22, wherein the at least one stop directly contacts atleast a portion of the flange.
 24. The gas turbine engine of claim 22,wherein the flange includes a flared cross section when viewed along acircumferential plane intersecting the flange.
 25. The gas turbineengine of claim 22, wherein the flange includes a U-shaped cross sectionwhen viewed along a radial plane intersecting the flange.
 26. The gasturbine engine of claim 21, wherein the at least one stop includes afirst stop confronting a first portion of the outer nozzle end and asecond stop confronting a second portion, different from the firstportion, of the outer nozzle end.
 27. The gas turbine engine of claim26, wherein the first stop is configured to axially position the nozzlestage and the second stop is configured to circumferentially positionthe nozzle stage.
 28. The gas turbine engine of claim 26, wherein thefirst stop extends from the second stop.
 29. The gas turbine engine ofclaim 21, further comprising an outer casing encasing at least a portionof the shroud.
 30. The gas turbine engine of claim 29, wherein the atleast one stop extends radially inwardly, with respect to thelongitudinal axis, from the outer casing.
 31. The gas turbine engine ofclaim 21, further comprising a shroud seal provided between the nozzlestage and the shroud.
 32. The gas turbine engine of claim 31, whereinthe shroud seal is provided between the outer nozzle end and the shroud.33. The gas turbine engine of claim 31, further comprising a pocketformed within a portion of one of either the nozzle or the shroud,wherein the seal is at least partially positioned within the pocket. 34.A shroud for an airfoil assembly having an airfoil with an outer end,the airfoil assembly configured to rotate about a rotational axis, theshroud comprising: a forward end; and an aft end positioned axially aftof the forward end with respect to the rotational axis, the outer end ofthe airfoil assembly extending through the aft end such that the outerend extends radially outward beyond a radially outer surface of theshroud; wherein at least one stop confronts at least a portion of theouter end and is configured to axially or circumferentially position theairfoil assembly with respect to the rotational axis.
 35. The shroud ofclaim 34, wherein the outer end includes a flange extending across atleast a portion of the outer end.
 36. The shroud of claim 35, whereinthe at least one stop directly contacts at least a portion of theflange.
 37. The shroud of claim 34, wherein the at least one stopincludes a first stop confronting a first portion of the outer end and asecond stop confronting a second portion, different from the firstportion, of the outer end.
 38. The shroud of claim 37, wherein the firststop is configured to axially position the airfoil assembly and thesecond stop is configured to circumferentially position the airfoilassembly with respect to the rotational axis.
 39. The shroud of claim37, wherein the first stop extends from the second stop.
 40. The shroudof claim 34, wherein an outer casing encases at least a portion of theshroud and wherein the at least one stop extends radially inwardly fromthe outer casing, with respect to the rotational axis.